The typical structure of a CD-R or DVD-R optical memory disc includes an energy absorbing dye layer formed directly over an injection molded polymeric substrate. Next, a thin film reflective layer is formed directly over the dye layer, usually via magnetron sputtering. Finally, a radiation curable protective lacquer is spin coated directly over the reflective layer. The reflective layer also functions as a barrier to isolate the dye layer from the uncured protective lacquer.
In most optical memory disc structures, dielectric layers are also formed between the active recording layer and adjacent layers, in order to protect the substrate and reflective layers from exposure related damage, and to encapsulate the active recording layer, protecting it against chemical contamination and long term deterioration. In a typical CD-R or DVD-R disc, these additional layers are omitted to reduce manufacturing cost. The dye (active recording layer) is coated directly onto the surface of the polymeric substrate, and the reflective layer is formed directly over the dye layer. Consequently the dye/substrate interface, and the dye/reflective layer interface are directly effected by the recording process. Because CD-R and DVD-R are "write once" formats, cumulative degradation does not develop as it would in a re-writable optical memory disc. However, cumulative degradation is not the only concern. The physical and chemical characteristics of the interface between the dye and substrate, and the dye and the reflective layer effect the exposure and read back performance of the optical memory disc. For example, the energy associated with the exposure process effects the surface of the polymeric substrate at the interface with the active recording layer. This causes an intermixing of decomposed dye and polymer. The degree of intermixing is related to the time at temperature profile created during exposure. When a recording format with a large range of pulse widths is employed, as is the case with CD-R and DVD-R, the longer pulses tend to be distorted by this mechanism. Another problem is distortion of the thin film reflective layer by the heat and pressure created during exposure. This distortion extends beyond the location of the intended mark, and has the effect of increasing the apparent size of the recorded mark. The negative effects of this distortion are most apparent when the spaces between recorded marks are short. Mark length deviation, inter-symbol interference, and jitter performance are all negatively effected by this "doming" effect. The exact profile of this distortion depends on the physical properties of the reflective layer, adhesion between the reflective layer and the underlying layer, and the characteristics of any layers coated on top of the reflective layer, such as a protective lacquer or bonding adhesive.
Reflective films thicker than approximately 100 nm tend to resist distortion caused by the exposure process. However, thick reflective layers typically exhibit higher stress which can lead to a variety of long term reliability problems. Additionally, the high thermal conductivity of typical metallic reflective films effects overall record sensitivity, and alters the thermal profile that develops during the formation of individual recorded marks. Consequently, reflective layer thickness cannot be arbitrarily changed in order to minimize physical distortion.
Disc manufacturing problems also result from inadequate preparation of the polymeric substrate. Due to well-known variables associated with the injection molding of plastics, the surface characteristics of the substrates are not absolutely constant. Without some means to provide a uniform and consistent surface condition, these variations in the substrate will effect the dye coating process and, therefore, achievable disc performance.
Further, the solvents used to prepare the dye for spin coating extract volatiles from the plastic substrate during the spin coating operation. This results in non-uniform spin coating performance, and contamination of the excess dye solution that is spun off of the disc. Typical substances that are leached out of the substrate include stabilizers, molding release agents, degraded polymer, residual monomer (etc.), and various adsorbed contaminants from the local environment. Correcting this situation will improve dye coating consistency, and increase manufacturing yield. Reducing contamination of the excess dye solution will allow the dye to be recycled more times, further reducing manufacturing costs.
The traditional solution to this problem would be to vacuum coat a thin film dielectric barrier onto the substrate prior to spin coating the dye. Unfortunately, this approach is complex and has the added disadvantage of high initial equipment cost.
Alternatively, treating the substrate with ultraviolet light, or a combination of ultraviolet light and ozone, will create a clean, uniform surface. Unfortunately, this is only a surface effect and does not reduce the tendency for volatiles to be leached out of the substrate by the solvents used in the dye solution.
Pre-rinsing the substrate surface with a solution that extracts volatiles, dilutes them, and washes them away will tend to reduce subsequent dye solution leaching of these substances. However, pre-rinsing the substrate surface with a buffered solvent will not guarantee a uniform surface with a stabilized, low level of extractables.
Therefore, it has been found desirable to provide an economical, pre-treatment/pre-coating layer between the disc substrate and the active dye layer of an optical memory disc which not only extracts volatiles, dilutes them, and washes them away, but also provides a uniform barrier to subsequent leaching by the solvents used for the dye solution. Moreover, it has been found desirable to provide a protective barrier layer which does not significantly alter the optical tuning of the disc structure, in order to minimize manufacturing complexity and cost.